The present invention relates to the field of electrostatographic image recording, such as electrophotography and electrography. More particularly, it relates to improvements in apparatus and methods for neutralizing electrostatic charges retained on image-receiving members and image-recording elements as a result of the electrostatographic image-forming process.
In the electrostatographic image recording process, an electrostatic charge image is formed on the photoconductive or dielectric surface of an image-recording element. The charge image is rendered visible by the application of pigmented electroscopic particles commonly known as "toner". To enable re-use of the image-recording element, the toner image is transferred to image-receiver member, typically a sheet of paper, which is brought into contact with the toner-bearing recording element at an image-transfer station. At this station, it is common to subject the receiver sheet to an electric field tending to attract the toner from the recording element to the receiver. Alternatively, toner transfer may be effected by the application of heat and/or pressure to the receiver sheet. Of course, any untransferred or residual toner on the recording element must be removed or cleaned from the recording element prior to recycling the recording element through the image-forming process.
In many electrostatographic processes, it is advantageous to neutralize any electrostatic charges remaining on untransferred toner particles prior to the aforementioned cleaning step. Such charge neutralization is typically effected by an AC corona charger which operates to alternately deposit negative and positive ions on the residual toner, the effect being that the net charge is driven to zero.
Once toner has been successfully transferred to a receiver sheet, it is common to fuse the toner to the receiver by the application of heat and/or pressure. Usually, the receiver sheet exits the fusing process with a net charge, whereby the sheet has a propensity for being attracted to other sheets or working surfaces. The net charge on the receiver sheet is due to the residual charge of the toner on its surface, as well as the charge that may have been applied to the sheet to effect toner transfer. The effect of toner charge is magnified in non-electrostatic transfer processes (e.g., thermal and/or pressure transfer processes) where the toner never comes in contact with an oppositely charged receiver sheet. The problem of static attraction of the receiver sheet is often handled by static discharge brushes and the like. The effectiveness of such devices is dependent on the conductivity of the receiver and toner. Overhead transparencies are usually the most difficult to discharge due to their insulative properties.
In conventional electrostatographic image recording apparatus, the above noted charge-neutralizing functions are provided by separate and distinct charge neutralizers, one being used to discharge the charge on the untransferred toner, and the other being used to discharge the residual charge on the receiver sheet following image transfer. The need for two charge neutralizers adds expense and complexity to the image-recording apparatus.